Existing Technologies for Thermal Remediation
Several ex-situ technologies have been employed to vaporize contaminants from materials. Techniques that have been employed include thermal desorption or remediation as exemplified by U.S. Pat. No. 5,904,904, U.S. Pat. No. 5,710,360, U.S. Pat. No. 5,656,178, U.S. Pat. No. 5,514,286, U.S. Pat. No. 5,361,514, U.S. Pat. No. 5,242,245 and U.S. Pat. No. 5,188,041.
U.S. Pat. No. 5,904,904 presents a thermal desorption treatment technique for the removal of contaminants from particulate material using an inclined rotating drum similar to a rotary dryer or kiln. Removal of the contaminants moving through the system by rotating the kiln is accomplished by vaporizing the components in an air environment by the counter-current flow of air relative to the material employing both direct and indirect heat. A burner firing into the center of the drum applies direct heat. Process temperature is controlled with excess air. Hot products of combustion are passed through a set of fire tubes providing indirect heat to the process.
U.S. Pat. No. 5,710,360 presents a system that employs an inductive heating system tube surrounding a single screw conveyor for decontaminating materials. The tube is designed to withstand temperatures as high as 982° C. (1800° F.).
U.S. Pat. No. 5,656,178 presents a method for the thermal desorption of contaminated materials using superheated steam, acidification, and fluidization in a batch process. In this process the superheated steam is recycled with any water in the feed material being converted into superheated steam.
U.S. Pat. No. 5,514,286 presents a thermal desorption method and apparatus using a vacuum batch process for removing chemical contaminants from soil. The method employs a vacuum batch process where the vessel is a modification drum of a cement truck with ribbon flights.
U.S. Pat. No. 5,361,514 presents a method that removes volatile materials by thermal desorption in a controlled oxygen environment by employing a flat type belt conveyor with counter current gas flow over the top of the material on the belt.
U.S. Pat. No. 5,242,245 presents a method and apparatus for vacuum enhanced thermal desorption in a device that employs two concentric rotating cylinders. The contaminated material is desorbed and conveyed parallel to the cylinder axes through the annular space. Heat is supplied to the material from a centrally mounted heat source located inside the inner cylinder.
German Patent DE 405,067B1 relates to a device for removing volatile noxious substances from soil and similar materials and is directed to creating a device that allows for decontamination with a low energy loss. Contaminated soil is excavated for treatment and then ventilated a gas such as air, whereby the volatile noxious substances are desorbed from the surfaces of material particles and stripped from most material components. Only low temperature treatment is discussed, and higher temperatures are prohibitive given the injection of air.
EP 327, 896B1 describes a method of cleaning earth-like materials that includes reducing the material to a size of less than or equal to 40 mm with the addition of water and a solvent or a mixture of solvents.
U.S. Pat. No. 5,178,077 discloses an apparatus and method for the removal of higher and lower volatility organic contaminants from media. The apparatus includes an externally heated screw conveyor having one or more solid core flights. The media within the screw conveyor is heated to a temperature above which at least one of the contaminants volatilize. The volatilized contaminants are then vented from the screw conveyor. External heating is provided by at least one infrared radiation source external to the screw conveyer.
U.S. Pat. No. 4,917,023 discloses a system for fixing, encapsulating, stabilizing and detoxifying contaminants using a two zone pyrolysis. The first zone is a drying zone and the second is a stabilization zone. The zones are brought to temperature using infrared heaters, a baffle separates the two zones, a condenser receives effluent from the drying zone and an afterburner receives effluent from the stabilization zone. To aid in the pyrolysis of metal-containing feed material, this material is mixed with a carbon source to provide direct contact of the metals in the feed with a sufficient amount of a carbon source to achieve a fixation step.
U.S. Pat. No. 5,988,947 presents a multi-chambered soil remediation device that can be adapted to mount to a vehicle wherein each chamber has an inlet, an outlet and a soil conveyor. The soil conveyor of one chamber feeds material into the inlet of a subsequent chamber. The soil is treated as it is conveyed within the chambers by the injection and removal of materials.
U.S. Pat. No. 6,146,596 presents a method of soil remediation that includes heating and vaporizing contaminated compounds. A blower is used to withdraw the organic hydrocarbon gases and other vaporized contaminants through the soil and out to the condenser where the compounds are liquified and containerized.
U.S. Pat. No. 4,738,206 presents an apparatus and method for low temperature thermal stripping of volatile organic compounds from soil. The invention includes sealing the soil in a stripping conveyor against contact with air and vapor stripping the contaminants at a temperature below the boiling temperatures of the contaminants.
Limitations of Existing Technologies
Low temperature thermal desorption has process temperatures ranging from 90 to 316° C. (200 to 600° F.) in contrast to high temperature thermal desorption where temperatures range from 316 to 427° C. (600 to 800° F.). The desorption methods are commonly used to remove volatile and semi-volatile organics from a matrix material. These techniques are not effective in removing high boiling point organics such as coal tars or asphalt. The two common thermal desorption designs are rotary dryers and thermal screws. These desorption methods are described by the above-mentioned patents. These patents demonstrate the use of systems for thermal desorption that employ rotary dryers or kilns, high temperature belt conveyors and screw conveyors. Some of these patents also demonstrate both direct and indirect methods of heat transfer. Process conditions for these processes vary widely, with some indicating vacuum to near atmospheric pressures for separating organic contaminants and temperatures from ambient to 982° C. (1800° F.). However, if air or oxygen are introduced, this allows for the creation of CO and CO2 by oxidation. Oxidation also changes the composition of the vaporized and non-vaporized organic contaminants present in the media being processed.
Pyrolysis is the chemical decomposition of organic materials induced by heat in an inert environment. Common names are molten solid processing or plasma pyrolysis. Organic materials are transformed into gas and a solid material containing fixed carbon and ash. Pyrolysis occurs at temperatures greater than 430° C. (800° F.). Common pyrolysis methods include rotary kilns, rotary furnaces and fluid bed furnaces. Pyrolysis is described by U.S. Pat. No. 4,917,023.